Arctic Essays

Ocean Acidification

J. T. Mathis1, J. N. Cross2

Highlights

The waters of the Arctic Ocean are disproportionately prone to ocean acidification compared to the rest of the global ocean due to the intensity, duration and extent of natural and anthropogenic drivers.

Over the next 2-3 decades, it is likely that ocean acidification will continue to intensify, especially over the shallow Arctic shelves. The rapid rate of this change is likely to have detrimental effects on ecosystems that are already under pressure from rising temperatures and other climate-driven stressors.

Over the past five years, ocean acidification (OA) has emerged as one of the most prominent issues in marine research. This is especially true given the newfound public understanding of the potential biological threat to marine calcifiers (e.g. clams, pteropods) and associated fisheries, and the associated human impacts for small communities that directly or indirectly rely on them (e.g., Mathis et al., 2015a; Frisch et al., 2015). Cooler water temperatures and unique physical processes (i.e. formation and melting of sea ice) make the waters of the Arctic Ocean disproportionately sensitive to OA when compared to the rest of the global ocean. Even small amounts of human-derived carbon dioxide (CO2) can cause significant chemical changes that other areas do not experience, and these could pose an existential threat to some biological organisms.

Current data indicates that certain areas of the Arctic shelves presently experience prolonged ocean acidification events in shallow bottom waters (Bates, 2015; Cross et al., 2016). Because natural carbon accumulation is already high near shelf sediments, these waters are most vulnerable to anthropogenic CO2 accumulation. These waters are eventually transported off the shelf. As a result, new analysis released this year suggests that corrosive conditions have been expanding deeper into the Arctic Basin over the last several decades (Qi et al., 2016). These Pacific-origin corrosive waters have been observed as far as the entrances to Amundsen Gulf and M'Clure Strait in the Canadian Arctic Archipelago (Mathis et al., 2012; Cross et al., 2016). The formation and transport of corrosive waters on the Pacific Arctic shelves may have widespread impact on the Arctic biogeochemical system, compounding acidification all the way to the North Atlantic.

The inherently short Arctic food web linkages may lead to widespread impacts of OA on the Arctic marine ecosystem. For example, many upper-trophic level organisms (e.g. walrus, grey whales and salmon) rely on the marine calcifiers most likely to be impacted by OA (Cross et al., 2016). Juvenile and larval life stages of these upper trophic organisms are also particularly vulnerable to OA (e.g., crabs [Long et al., 2013a,b; Punt et al., 2014] and shellfish, [Ekstrom et al., 2015]). In turn, many subsistence communities rely on these upper trophic populations. While biological impacts of OA are not presently visible, it is likely that OA conditions will intensify over the next 2-3 decades and may produce more prominent impacts (Mathis et al., 2015b; Punt et al., 2016). Present-day acidification hotspots are produced by a combination of natural CO2 accumulation caused by biological cycles every season with additional, human-derived CO2 absorbed from the atmosphere. Natural CO2 accumulation tends to be highest in areas where currents are particularly slow, and where bottom waters are largely isolated from the surface. As anthropogenic CO2 continues to increase, the Arctic Ocean will be pushed past important chemical thresholds even in areas where natural carbon accumulation is lower (Mathis et al., 2015b).

In the last twelve months, several comprehensive data synthesis products (Mathis et al., 2015b; Bates, 2015; Semiletov et al., 2016; Qi et al., 2016; Cross et al., 2016) have been published using much of the available OA data that has been collected in the Arctic Ocean. Several trends have emerged that clearly elucidate the rapid progression of OA across the Arctic basin, including rapid CO2 uptake from the atmosphere and increasing carbonate mineral corrosivity (e.g., Evans et al., 2013). These trends are compounded by regional variability that is controlled by a number of physical and chemical processes including the rate of air-sea gas exchange of carbon dioxide (e.g. Bates, 2015; Evans et al., 2015); upwelling (Mathis et al., 2013); transport of allocthonous terrestrial carbon from river discharge and permafrost degradation (Semiletov et al., 2016); sea-ice melt (Yamamoto-Kawai et al., 2013); and respiration (Bates 2015).

Additionally, the indirect effect of changing sea-ice coverage is providing a positive feedback to OA (see essay on Sea Ice). For example, the reduction in Arctic and sub-Arctic sea ice observed in recent years can be attributed to increased warming caused at least in part by rising atmospheric CO2 levels. The reduction in the extent and duration of sea-ice cover leads to longer open water periods, allowing for enhanced upwelling and changes in the timing and intensity of primary production (see essay on Primary Productivity) and sea-air gas exchange of CO2 (e.g. Evans et al., 2015). Combined with the fact that cooler temperatures and global ocean circulation processes precondition the continental shelves in this region to have relatively low carbonate mineral saturation states compared to the rest of the ocean, these direct and indirect acidification processes make the region highly vulnerable to further reductions in seawater pH and saturation states.

Though the specifics remain uncertain, it is likely that the consequences of continuing OA will be detrimental for parts of the marine food web (Mathis et al., 2015a). Continued monitoring of OA in the Arctic Ocean is essential to understand the ecosystem transitions currently underway due to the suite of anthropogenic pressures. The Arctic region provides unique insights into how the global ocean will respond to human activities and it is our best hope for developing the understanding that will be needed to adapt to what will be our new, modern ocean environment.

References

Bates, N. R. 2015. Assessing ocean acidification variability in the Pacific-Arctic region as part of the Russian-American Long-term Census of the Arctic, Oceanography 28(3):36-45, http://dx.doi.org/10.5670/oceanog.2015.56.